The current invention concerns an apparatus for aligning a blade ring to the cylinder of a steam turbine. More specifically, a tool for rotating and handling an alignment dowel during its installation into the steam turbine cylinder is provided.
In a steam turbine, the stationary vanes are fitted into circular rings, referred to as blade rings, housed within the turbine cylinder. In addition to holding the vanes, the blade rings form a shroud over the tips of the rotating blades. In order to prevent steam from leaking around the row of rotating blades, which would reduce the energy extracted from the steam, there is little radial clearance between the blade ring and the tips of the rotating blades. Moreover, during operation, differential thermal expansion between the stationary and rotating components reduces the radial clearance further. Thus, in order to prevent the rotating blade tips from contacting the blade ring, it is important that the blade ring be carefully aligned with respect to the rotor. Since the rotor is centered in a turbine cylinder, this means that the blade ring must be accurately aligned to the turbine cylinder.
Each blade ring is aligned to the turbine cylinder in the transverse direction by radial dowels at the 12 o'clock and 6 o'clock locations. The dowels are retained in close fitting holes in the turbine cylinder. The inboard end of each dowel features a key which inserts into a slot in the periphery of the blade ring. Alignment is achieved by specially machining each key so that, when inserted into the slot, the key locates the blade ring in its proper alignment. This alignment is maintained by welding the dowel to the turbine cylinder.
Although a close fit of the dowel key within the slot in the blade ring is desired in order to maintain accurate alignment, there is a clearance of approximately 0.076 mm (0.003 inch) between each side of the slot and the key. To ensure that the blade ring is properly aligned, it is important that the key be situated so that its faces are parallel to the sides of the slot. However, as a result of the clearance, it is possible to rotate the key slightly in the slot so that its faces are not parallel to the sides of the slot--that is, the key can be cocked, with opposing corners of the key contacting the sides of the slot. To ensure the dowel is not welded to the turbine cylinder with the key in this cocked position, a centering operation is performed during the assembly process. This centering operation involves rotating the dowel clockwise until contact with the sides of the slot is felt and scribing lines on the dowel and the turbine cylinder marking this orientation. The dowel is then rotated counterclockwise until contact is felt in that direction and a second line is scribed on the turbine cylinder in line with the line previously scribed on the dowel. The dowel is then rotated so that its scribe line is midway between the two scribe lines on the turbine cylinder and welded to the cylinder in this position.
Note that because of the close fit between the dowel and its retaining hole in the turbine cylinder, along with the presence of dirt and burrs in the hole, rotating the dowel during the assembly procedure often requires considerable torque. However, the method of applying torque must provide sufficient "feel" to enable the assembler to determine when contact between the key and the slot has occurred during the scribing operation.
The assembly procedure discussed above places four requirements on the dowel which must be addressed during its design: (i) it must be capable of being lifted and held while being inserted into the turbine cylinder, (ii) it must facilitate the application of substantial torque to turn the dowel, (iii) it must allow the torque to be applied in a manner which provides sufficient "feel" to determine when the key has contacted the sides of the slot, and (iv) it must allow the dowel to be accurately held in place during the scribing operation. Since the body of the dowel is round, it does not lend itself to the application of conventional box or open end wrenches. Although a pipe wrench is capable of supplying the necessary torque to turn the dowel, it would not satisfy requirements (iii) or (iv) above because of slippage inherent in the use of such wrenches. According to the prior art, the problem of wrench engagement was overcome by the use of an integral stub emanating from the outboard end of the dowel. Flats were milled on opposing surfaces of the stub to enable engagement by an open end wrench. In addition, a hole was drilled and tapped in the end of the stub to allow the attachment of a handle which facilitated lifting and handling of the dowel during assembly. Although this dowel design performed satisfactorily, as explained below, other constraints in the dowel design made the manufacturing of this integral stub undesirably expensive.
As previously mentioned, the dowels are retained by welding them to the turbine cylinder. A closure weld bead is placed around the periphery of the dowel where it projects through the turbine cylinder. In operation, substantial loads are transmitted to the weld bead from the turbine cylinder and the dowel, due to internal pressure in the turbine cylinder and differential thermal expansion between the blade ring and the turbine cylinder. In order to ensure that these loads do not result in the cracking of the closure weld, the dowel is made flexible in the vicinity of the weld, thereby reducing the loading imparted to the weld. This flexibility can be achieved by hollowing out the dowel at its outboard end--that is, in the portion of the dowel enclosed by the closure weld. In order to retain the integral stub, this hollowing out was limited to machining a circular groove in the end face of the dowel concentric of the center line of the dowel. The circular groove reduced the stiffness of the dowel to essentially that of a tube in the vicinity of the weld. The portion of the dowel remaining inside the circular groove formed the integral stub.
Machining of the circular groove requires an operation called trepanning, in which a cutting tool, attached to a rotating hollow cylindrical head, is fed into the dowel. Trepanning is a difficult and costly operation and results in frequent breakage of the tool. This is especially so in this case because the groove must often be two inches deep to obtain the necessary flexibility. Thus, as a result of the trepanning operation and the separate milling operation to produce the flats on the stub, the cost of manufacturing the dowel is extremely high.
Consequently, it is desirable to provide a means for rotating and handling the dowel which allows the required flexibility to be incorporated into the portion of the dowel in the vicinity of the weld, without the need for the expensive machining operations associated with the integral stub.